1. Field of the Invention
This invention relates to a motion sensor worn by a person having at least one mercury switch for detecting motion of the person to initiate an alarm when normal motion ceases.
2. Description of Related Art
Personal alert devices (often called "PASS" units in the industry) are known which are worn by persons working in hazardous environments. In a hazardous environment, a worker might be overcome by elements in their environment such as smoke, fumes, heat, electrical shock, or falling debris. It is desirable to sense when a person has been disabled or is in duress and to generate a warning signal so that the person can be located and rescued. Typical users of such personal warning devices include firemen, petrochemical workers, miners, electricians, sewage engineers, nuclear power plant workers, and military personnel.
While such personal warning devices are known, all such current devices are deficient in a number of respects. The most typical problems of known personal warning devices are that they either initiate a false alarm when the wearer is continuing normal activity, or they fail to initiate an alarm when the wearer becomes disabled. Further, all of these personal warning devices are unable to distinguish between many types of normal working activity and duress situations.
Most known personal alarm systems use one of two types of motion sensors. The first type of motion sensor uses a cantilevered lever which is fixed at one end and has a contact at the movable end. The lever may be either a rigid clapper or a spring. A mating contact surrounds the movable end of the cantilever lever. Thus, normal motion is supposed to periodically establish contact between the movable end and surrounding mating contact. This design must have a sufficiently rigid lever that when oriented in various axes the lever does not establish electrical contact due to gravity and inadvertently become inoperative (i.e. giving a false indication of normal activity). Unfortunately, such a rigid lever prevents detection of slow changes in motion (possibly normal activity). The spring-type of cantilevered lever often fatigues with time and use, thus changing its operating characteristics. The spring-type of lever also does not provide uniform repeatable contact closure resistance and severe shock to motion sensor can cause the spring or its mating contact to be physically damaged.
The second common type of motion sensor incorporates a movable ball which rolls in an enclosure such as a cylinder or sphere. Wearer motion is detected when the ball shorts electrical contacts in the enclosure as it rolls about. A variation of this type of motion sensor fixes a piezoelectric material to the enclosure such that electrical voltages are produced when the ball moves. Such ball sensors are very sensitive to small movements and do not provide an easy mechanism for discriminating between normal motion and a possible duress situation.
As can be appreciated, known personal warning systems all incorporate motion sensors which are not always reliable and dependable. Further, such personal warning systems are largely ineffective in distinguishing between many types of normal activities and duress situations.